A typical optical module in which an optical fiber and a laser diode are optically coupled with each other is disclosed in Japanese Published Patent Application No. Hei. 7-43565. In the optical module disclosed in this publication, a substrate having a V groove on its upper surface is employed, and an optical fiber is disposed in the V groove. A laser diode is disposed on the upper surface of the substrate outside the V groove so that a light emitting part of the laser diode faces a core of the optical fiber. In this optical module, since the laser diode is fixed directly onto the upper surface of the substrate, when the laser diode is fabricated so that the distance from the lower surface of the laser diode to the light emitting point is equal to the distance from the upper surface of the substrate to the core of the optical fiber, alignment of the laser diode with the optical fiber is accurately performed in the direction perpendicular to the surface of the substrate (hereinafter referred to as height direction).
Meanwhile, in an optical module employing a photodiode, when the photodiode is a facet incidence type having a light responsive part on its facet, this photodiode is fixed directly onto the upper surface of the substrate as in the above-mentioned optical module including the laser diode, whereby alignment between the photodiode and the optical fiber in the height direction is accurately performed. However, when a surface incidence type photodiode having a light responsive part on its surface is employed, since the photodiode must be disposed so that the light responsive part is perpendicular to the optical axis of the optical fiber or a little off the direction, a block for fixing the photodiode is usually used.
FIGS. 15 and 16(a)-16(c) are diagrams illustrating a conventional optical module in which a surface incidence type photodiode and an optical fiber are optically coupled with each other.
The optical module includes a photodiode 1, a block 2 on which the photodiode 1 is fixed, an optical fiber 3, a substrate 4 having a V groove 41 in which the optical fiber is fixed (hereinafter referred to as a V-groove substrate), a cover 5 having a V groove 51 (hereinafter referred to as a V-groove cover), a package 6, and a cover 7 of the package 6. In the optical module, the block 2 on which the photodiode 1 is fixed and the V-groove substrate 4 on which the optical fiber 3 is fixed are arranged in the package 6 so that an end facet of the optical fiber 3 is opposed to a light responsive surface of the photodiode 1.
As shown in FIG. 16(c), the photodiode 1 has a light responsive part 11 in the center of the upper surface, and the light responsive part 11 receives a light beam emitted from the optical fiber 3. The photodiode 1 is fixed onto a side surface of the block 2 and connected to an electrode 2a on the block 2 with a feeding wire 2c. The electrode 2a on the block 2 is connected to an electrode of the package 6 with feeding wires 2b.
As shown in FIGS. 16(a) and 16(b), the optical fiber 3 comprises a core 31 and a cladding 32 surrounding the core 31. Since the refractive index of the core 31 is higher than the refractive index of the cladding 32, light incident on the optical fiber 3 is guided through the core 31. The optical fiber 3 is sandwiched between the V-groove substrate 4 and the V-groove cover 5 and fitted in the V groove 41 of the substrate 4 and the V groove 51 of the cover 5. Since the V groove 41 of the substrate 4 is formed by photolithography, the width and the depth of the V groove 41 are controlled with an accuracy better than 1 .mu.m. Hence, when the optical fiber 3 is fitted in the V groove 41, the distance between the core 31 in the center of the optical fiber 3 and the upper surface of the V-groove substrate 4 is determined with high accuracy.
A description is given of the method for fabricating the optical module. FIG. 20 is a flow chart illustrating process steps in the method of fabricating the optical module.
Initially, in step S1, positioning of the photodiode 1 in the height direction (y direction) is performed by pictorial pattern recognition so that the distance t.sub.2 from the light responsive part 11 of the photodiode 1 to the rear surface 2d of the block 2 is equal to the sum of the thickness t.sub.1 of the V-groove substrate 4 and the distance from the core 31 of the optical fiber 3 to the surface of the substrate 4, followed by fixing the photodiode 1 onto the side surface of the block 2 with Au or Sn solder, or a resin. In step S2, the photodiode 1 is connected to the electrode 2a on the block 2 with the feeding wire 2c. Thereafter, in step S3, the V-groove substrate 4 is fixed onto the package 6 with a resin or the like. Next, in step S4, positioning of the block 2 in the horizontal direction (x direction) is performed by pictorial pattern recognition so that the light responsive part 11 of the photodiode 1 on the block 2 is opposed to the center of the V groove 41 of the substrate 4, followed by fixing the block 2 onto the package 6 with a resin or the like. In step S5, the optical fiber 3 is fitted in the V groove 41 of the substrate 4. In step S6, the V-groove cover 5 is fixed on the V-groove substrate 4 so that the optical fiber 3 is fitted in the V groove 51 of the cover 5. In step S7, the electrode 2a on the block 2 is connected to the electrodes of the package 6 with the feeding wires 2b. Finally, in step S8, the package 6 is sealed with the package cover 7, completing the optical module shown in FIGS. 15 and 16(a)-16(c).
FIGS. 17 and 18 are a perspective view and a cross-sectional view illustrating an optical module in which a surface incidence type photodiode and an optical fiber are optically coupled with each other, according to a prior art.
This optical module is similar to an optical module disclosed in "PD Module with Pre-Amplifier on an Si Platform", Extended Abstracts, 1996 General Meeting of Institute of Electronic Information Communication, (p219:C-219). In this optical module, a block for fixing a photodiode as the block 2 used in the optical module shown in FIG. 15 is not used, and a photodiode 1 is disposed directly on a V-groove substrate 8, whereby the photodiode 1 is optically coupled with an optical fiber 3. More specifically, as shown in FIGS. 17 and 18, a substrate 8 has a V groove 8a and a hole 9 connected to the V groove 8a. The optical fiber 3 is fitted in the V groove 8a with an end portion protruding into the hole 9, and the photodiode 1 is disposed on an inclined side wall 9a in the groove 9, facing the optical fiber 3. Since the photodiode 1 is disposed on the inclined surface 9a, a light responsive surface 11 of the photodiode 1 is inclined in the height direction, i.e., it is not perpendicular to the optical axis of the optical fiber 3.
A description is given of the method for fabricating the optical module. FIG. 21 is a flow chart illustrating process steps in the method of fabricating the optical module.
Initially, in step S1, positioning of the photodiode 1 in the height direction is performed by pictorial pattern recognition so that the distance from the surface of the substrate 8 to the light responsive part 11 of the photodiode 1 is equal to the distance from the surface of the substrate 8 to the core of the optical fiber 3, and positioning of the photodiode 1 in the horizontal direction is performed by pictorial pattern recognition so that the light responsive part 11 of the photodiode 1 is opposed to the center of the V groove 8a, followed by fixing the photodiode 1 on the inclined surface 9a using a resin or the like. Then, in step S2, the photodiode 1 is connected to an electrode 2a on the V-groove substrate 8 with a feeding wire 2c. In step S3, the V-groove substrate 8 is die-bonded to a package (not shown) with a resin or the like. In step S4, the electrode 2a on the V-groove substrate 8 is connected to electrodes on the package with feeding wires 2b. Thereafter, in step S5, the optical fiber 3 is fitted in the V groove 8a of the substrate 8. Finally, in step S6, a V-groove cover (not shown) is fixed on the V-groove substrate 8, and the package is sealed with a package cover (not shown), completing the optical module.
Meanwhile, Japanese Published Patent Application No. Hei. 2-149805 discloses a method for coupling an optical fiber with an optical waveguide on a substrate, in which a waveguide substrate on which an optical waveguide is fabricated is connected to a connecting member on which an optical fiber is disposed, and the waveguide substrate and the connecting member are disposed on an upper surface of the substrate, with the surfaces of the waveguide substrate and the connecting member contacting the upper surface of the substrate, whereby the optical fiber is coupled with the optical waveguide. However, this coupling method is different from the coupling method of the optical module in which a photodiode fixed onto a block is optically coupled with an optical fiber, so that it is hard to say that this prior art method relates to the present invention.
A description is now given of optical fiber deviation vs. coupling efficiency characteristics between an optical fiber and a photodiode.
FIG. 19 is a graph illustrating optical axis deviations between an optical fiber and a photodiode which are separated by a distance of 50 .mu.m, which deviations are allowable for optical coupling between the optical fiber and the photodiode.
In FIG. 19, the ordinate shows the coupling efficiency (%), and the abscissa shows the optical axis deviation (.mu.m). A solid line, a dotted line, and an alternate long and short dash line show calculated values when the diameter of the light responsive part of the photodiode is 20 .mu.m, 30 .mu.m, and 40 .mu.m, respectively. Black dots, black triangles, and black squares show measured values when the diameter of the light responsive part is 20 .mu.m, 30 .mu.m, and 40 .mu.m, respectively.
As shown in FIG. 19, when the diameter of the light responsive part of the photodiode is 20 .mu.m, in order to secure a coupling efficiency of 90% between the optical fiber and the photodiode separated by a distance of 50 .mu.m, the allowable optical axis deviation is about 5 .mu.m.
In the prior art optical module shown in FIG. 15, even though the distance between the upper surface of the V-groove substrate 4 and the center of the optical fiber 3 is accurately determined, the coupling efficiency between the core 31 of the optical fiber 3 and the light responsive part 11 of the photodiode 1 varies according to the following factors:
1) the thickness of the V-groove substrate 4 supporting the optical fiber 3 PA1 2) the accuracy of the position (height) of the photodiode 1 when the photodiode 1 is fixed onto the block 2 PA1 3) the positioning accuracy when the block 2 is fixed onto the package 6
The thickness of the V-groove substrate 4 has a variation of .+-.10 .mu.m, and the accuracy in fixing the photodiode 1 on the block 2 has a variation of .+-.5 .mu.m. Therefore, an optical axis deviation of about .+-.15 .mu.m is produced between the optical fiber 3 and the photodiode 1 in the height direction (y direction). Further, since the accuracy in fixing the block 2 onto the package 6 has a variation of .+-.5 .mu.m, an optical axis deviation of about .+-.5 .mu.m is produced between the optical fiber 3 and the photodiode 1 in the horizontal direction (x direction).
Accordingly, in the optical module shown in FIG. 5, since the optical axis deviation between the photodiode 1 and the optical fiber 3 in the height direction (y direction) is about .+-.15 .mu.m, when the optical fiber 3 and the photodiode 1 are separated by a distance of 50 .mu.m and the diameter of the light responsive part of the photodiode 1 is 20 .mu.m or 30 .mu.m, it is difficult to achieve a coupling efficiency higher than 90%, as is evident from FIG. 19. That is, when the optical axis deviation is within .+-.5 .mu.m, a coupling efficiency higher than 90% is realized. Relating to the accuracy of the position (height) of the photodiode when it is fixed onto the block 2 and the positioning accuracy when the block 2 is fixed onto the package, the variations of these accuracies can be suppressed within .+-.5 .mu.m by highly accurate pictorial pattern recognition. However, relating to the thickness of the V-groove substrate 4 on which the optical fiber 3 is fixed, since a conventional Si substrate within a specified standard is usually employed, it is almost impossible to suppress the variation in the thickness of the substrate within .+-.5 .mu.m. As a result, in the prior art optical module shown in FIG. 15, it is difficult to accurately align the photodiode 1 with the optical fiber 3 in the height direction.
On the other hand, in the prior art optical module shown in FIGS. 17 and 18, although the optical coupling between the photodiode 1 and the optical fiber 3 is realized without being influenced by the thickness of the V-groove substrate 8, since the surface 9a of the substrate on which the photodiode 1 is disposed is inclined, formation of the electrode 2a on the inclined surface 9a, mounting of the photodiode 1 on the inclined surface 9a, and bonding of the feeding wire 2c to connect the photodiode 1 with the substrate 8 are complicated.